P079 First Results from Spaceborne Radar Interferometry for the Study of Ground Displacements in Urban Areas C.M. Crosetto (Instituto de Geomatica), C.A. Casas (University of Barcelona), R.G. Ranieri (University of Cagliari), L.F. Loddo* (University of Cagliari), A.I. Atzori (University of Cagliari) & M.M. Manunta (University of Cagliari) SUMMARY The Differential Synthetic Aperture Radar Interferometry (DInSAR) is a remote sensing technique that allows us to produce spatially dense deformation maps with centimetre to millimetre accuracy and to detect and follow the temporal evolution of deformations via the generation of time series. To do this, the information available from each interferometric data pair must be properly related to those included in the other acquisitions, via the generation of an appropriate sequence of DInSAR Interferograms. In this work we process a large set of data acquired by the European Remote Sensing (ERS) satellites on the Palau y Solitari de Plegamans area located near Barcelona (Spain) and on the Cagliari area (Italy). In the second work we apply an algorithm referred to as Small Baseline Subset (SBAS) proposed by I.R.E.A. C.N.R. The results demonstrate that the DInSAR technique is a fast and precise method to detect and follow the temporal evolution of deformations, so it is a powerful technique for monitoring the territory and to reduce its vulnerability.
Remote Sensing allows to study objects set to a great distance of the observer, through their interaction with the electromagnetic waves. In the actual panorama of remote sensing, Synthetic Aperture Radar (SAR) is an active, microwave imaging radar that can be installed on airplanes and satellites and allows the generation of high spatial resolution images of the observed scene. The product of the sensor acquisition is a complex image characterized by a module and a phase term. Differential Synthetic Aperture Radar Interferometry (DInSAR) is a technique that, by making the phase difference of SAR image pairs, allows to extract the information relevant to the earth surface displacements, to produce spatially dense deformation maps with centimetre to millimetre accuracy and to detect and follow the temporal evolution of deformations. It allows to study deformation phenomena without any previous information of the area, without any deformation model of the earth surface and with a very larger spatial density than of whatever GPS net. The result of the phase difference, referred to as Interferometric Phase or Interferogram, contains two principal terms: a topographic phase contribution and the deformation phase term. By removing the topographic component we are able to obtain the deformation of the observed scene. The removal of the topography can be carried out by using an external DEM of the site of interest (Fig.1). Fig. 0 Generation of differential SAR interferogram Since the differential interferogram represents a single event of deformation, in order to follow the temporal evolution of deformations, it is necessary a stack of differential interferograms (Fig.2). In fact, each differential interferogram represents an equation of a system which solution is the temporal evolution of deformations. Fig. 0 Reconstruction of displacements temporal evolution
In the first work we applied DInSAR methodology in the study of the Earth deformations in two urban areas subject to slow, but important displacements in the Valles zone (Fig.3) called Palau y Solitari de Plegamans located near Barcelona and characterized by the presence of a system of faults interesting a small town (Fig.4). Fig. 3 SAR amplitude image representing the studied area Fig. 4 Optical image of Palau city representing system of faults interesting the urban center Fig.5 shows fractures in walls and streets caused by the earth displacements. We process 18 images acquired by the ERS 1/2 satellites with a temporal baseline < 5 years obtaining a number of 46 interferograms. Fig.5 Damages on walls and streets in the small town interested by the subsidence phenomena Results are represented by velocity deformation maps of the studied area and by the mean rate deformation in the time interval from 1995 2000 (Fig.7). The observed deformations are calculated in the Line of Sight of the sensor and they are relative in the space (with respect to a point considered stable) and in the time (with respect to the first acquisition) (Crosetto et al., 2003).
Fig.6 Velocity deformation map superimposed on a SAR amplitude image of the studied area Fig.6 indicates the absence of deformation phenomena of regional character, and the presence of two local deformation areas located respectively near Palau and S. Perpetua cities. The first is characterized by a mean deformation velocity of about 4-5 mm/year while the second area, already studied in a past work, shows a mean deformation velocity of about 5-6 mm/year. Both deformation areas are just located on faults passing through the cities. It is moreover possible to see how the two deformation areas involve a wider zone located to the left of the two cities. The second work we present, is relative to the whole area near Cagliari (Italy) and we process a large set of data acquired from years 1992 2001 by the European Remote Sensing (ERS) satellites using the SBAS (Small Baseline Subset) approach proposed by I.R.E.A. - C.N.R. (Berardino et al., 2002; Lanari et al., 2004). The SBAS algorithm, that relies on the use of small baseline differential SAR interferograms, allows to generate deformation time series which are particularly indicated to study deformation phenomena in urban centres. The experiments have been developed on a dataset of 40 images acquired by the ERS 1/2 satellites, with spatial baseline less than 300 m and temporal baseline less than 4 years, with a number of 123 interferograms processed with the SBAS algorithm with a low resolution approach (Manunta et al., 2005). Every sampled point refers to an area of 80x80 meters. Obtained results indicate the absence of deformation phenomena of regional character, predictable result date the scarce seismic activity of the Sardinia region, and the presence of local deformations restricted in the Cagliari urban area, in particular in correspondence of the harbour area where silt and clay occur. (A) (A) (B) (B)
Fig.7 indicates how the whole area around the stadium is in lowering. Plots (A) and (B) clearly show the pattern of deformation with mean velocities respectively of -0,75 and -0,52 cm/year with a total displacement that, in less of 8 years, has reached respectively 5 and more of 6 cm. In both cases ground geophysical analysis (Electrical tomography, Gravity survey and Seismic) were made in order to verify the geological factors which cause deformations and some aspects seem to be in agreement with DInSAR results. The results achieved by applying the Differential Synthetic Aperture Radar Interferometry technique on the Barcelona and Cagliari case studies demonstrate that Differential Synthetic Aperture Radar Interferometry is a fast and precise method to detect and follow the temporal evolution of deformations, so it is a powerful technique for monitoring the territory and to reduce its vulnerability. References Berardino P., Fornaro G., Lanari R., Sansosti E., [2002] A new Algorithm for Surface Deformation Monitoring based on Small Baseline Differential SAR Interferograms. IEEE Transactions on Geoscience and Remote Sensing, Vol. 40, No. 11, pp. 2375-2383. Berardino P., Casu F., Lanari R., Manunta M., Manzo M., Pepe A., Sansosti E., [2004] A Small Baseline multi-platform DIFSAR approach. EUSAR 2004, Ulm (Germany), 25-27. Crosetto M., Tscherning C., Crippa B., Castillo M., [2002] Subsidence monitoring using SAR interferometry: reduction of the atmospheric effects using stochastic filtering. Geophysical Research Letters. Crosetto M., Castillo M., Arbiol R., [2003] Urban subsidence monitoring using radar interferometry: Algorithms and validation. Photogrammetric engineering and remote sensing. Crosetto M., Biescas E., Fernandez I., Torrobella I., Crippa B., [2003] Deformation control using SAR interferometry: quantitative aspects. Lanari R., Mora O., Manunta M., Mallorquì J.J., Berardino P., Sansosti E., [2004] A Small Baseline Approach for Investigating Deformations on Full resolution Differential SAR Interferograms. IEEE Transactions on Geoscience and Remote Sensing, Vol. 42, No. 7. Manunta M., Ranieri G., Loddo F., Lanari R., [2005] L Interferometria SAR Differenziale per il Monitoraggio delle Deformazioni della Superficie Terrestre: il Caso della Provincia di Cagliari, Proceedings GNGTS, pp 55-57. Manunta M., Ranieri G., Loddo F., Lanari R., [2005] On the Exploitation of Spaceborne Radar Interferometry for Earth Surface Deformation Analysis: the Cagliari Area Case Study, II International Conference of Applied Geophysics for Engineering, Messina, Italy October 13-16.